A vehicle may include a three-way catalyst (TWC) for treating exhaust gases of an internal combustion engine. Feedback control may be applied to regulate an engine's air-fuel ratio (AFR) so that engine exhaust constituents may be adjusted in a way that improves catalyst efficiency. Some vehicles may include a universal exhaust gas oxygen (UEGO) sensor positioned upstream of the TWC and a heated exhaust gas oxygen (HEGO) sensor positioned downstream of the TWC to control the AFR near stoichiometry. The UEGO sensor provides feedback to adjust engine out gases about stoichiometry. The HEGO sensor provides feedback to bias the engine AFR richer or leaner to improve catalyst efficiency.
An exhaust manifold comprises individual exhaust runners from each engine cylinder that collect into a single tube upstream of the catalyst. To minimize engine-startup emissions, the catalyst is placed as close to the cylinder exhaust ports as possible to quickly heat the catalyst. Meanwhile, the location of the UEGO sensor is optimized to measure the best mix of gases from each cylinder, given the limited available space. Since the exhaust tubing space upstream of the catalyst is limited, a typical issue that arises in naturally-aspirated engines is zone flow. Specifically, zone flow is an imbalanced rich/lean flow through the exhaust system resulting from limited space for exhaust gases to mix in a homogenous manner. If each cylinder AFR is matched with the other cylinders, this zone flow phenomenon is not an issue and stoichiometric AFR can be maintained. However, if there are AFR imbalances from cylinder to cylinder resulting from, say, part-to-part variability or intentionally-induced on-board diagnostics (OBD) imbalances, the exhaust stream will comprise differing levels of AFR depending on the location in the exhaust runner. If the UEGO sensor cannot measure a proper mix of each cylinder's gases due to this zone flow phenomenon, the rich/lean gases will quickly overwhelm the catalyst and exit the tailpipe as increased CO and NOx emissions.
Other attempts to address AFR imbalance include monitoring engine AFR using one or more HEGO and/or UEGO sensors. One example approach is shown by Behr et al. in U.S. Pat. No. 7,802,563. Therein, a method for monitoring AFR of an engine comprises routing exhaust gas from a group of cylinders to an oxygen sensor, sampling the oxygen sensor above a firing frequency of the group of cylinders, determining a difference between the samples over a window interval, and indicating an AFR imbalance in the group of cylinders when a ratio of at least the window interval over a total number of window intervals exceeds a threshold.
However, the inventors herein have recognized potential issues with such systems and methods. As one example, the system discussed above relies upon an individual exhaust gas sensor for each group of cylinders, some of which may be dedicated to AFR imbalance monitoring. As another example, if one or more of the exhaust gas oxygen sensors is degraded, the method may erroneously indicate an AFR imbalance due to biased functioning of the sensor. Further, if one or more of the exhaust gas oxygen sensors used to monitor engine AFR is downstream of a catalyst, the method may erroneously indicate an AFR imbalance due to degradation of the catalyst.
In one example, the issues described above may be addressed by a method comprising, during feedback engine air-fuel ratio control responsive to output of an exhaust gas sensor positioned downstream of a catalyst, indicating a cylinder imbalance responsive to a catalyst transfer function determined only within a specified frequency range based on the exhaust gas sensor output, and adjusting an actuator in response to the indicated cylinder imbalance. The method further comprises determining that the catalyst is nominal prior to indicating the cylinder imbalance. In this way, an air-fuel ratio imbalance may be accurately identified and mitigated, thereby reducing emissions.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.